The use of electroencephalogram information as an intraoperative monitor is well established in the medical literature. As noted by Levy, et al. Anesthesiology, Volume 3, September, 1980, pages 223-226), electroencephalogram (EEG) data is useful in monitoring during cerebrovascular surgery. Other relationships between the EEG data and cerebral ischemia, as well as central nervous system changes, have been noted.
Recent innovations in signal processing and display techniques for EEG data have increased the usefulness of this diagnostic tool in the operating room. Automated gain adjustment, automated electrode impedance checking and artifact detection relieve substantial amounts of the routine workload associated with operation of electroencephalographic monitoring equipment. Furthermore, concise displays conveying several minutes of EEG data aid in the interpretation of this data by physicians in the operating room.
Of these areas of advance in EEG technology, the display has been given much attention. Because of the relatively high volume of information which must be conveyed to the physician, and the period of time over which subtle changes in the presented information may occur, various presentation and display schemes have been developed in the prior art. In order to understand the various display schemes employed, a basic understanding of the nature of the EEG signal is necessary.
The electroencephalographic signal has typically been recorded as a voltage on a strip chart recorder. The wave form thus recorded displays several important frequency components. In addition, various artifacts are displayed which change the minute voltages being recorded. EEG activity commonly is in the range of 10 to 25 microvolts and is thus sensitive to changes in external conditions. In addition, voluntary and involuntary muscle response, and central nervous system changes, significantly affect EEG readings.
A problem with the strip chart method of recording EEG data arises because subtle changes in the baseline may be separated by 20 or more pages of paper tracings and thus be difficult or impossible for the physician to detect.
In an effort to achieve more useful data displays, the prior art has developed various data transformations. The Compressed Spectral Array (CSA) (shown in FIG. 2A) employs a plot of power versus frequency for discrete time intervals or epochs of analysis. The plotted power/frequency curves are then processed in a hidden line removal algorithm in order to provide a "hill-and-valley" display. Hills representing high power at a given frequency may be interpreted as they change in contour and location on the surface described by a series of plots. Typically, series of power/frequency lines is displayed on a cathode ray tube or printed on a paper chart. Typical of commercial analyzers using the CSA technique are the Model 1263 Berg Fourier Analyzer marketed by OTE Biomedica, Pathfinder by Nicolet, EEG Trend Monitor by Nihon Kohden, and Neurotrac.TM. from Interspec, Inc., the assignee of the present application.
The CSA technique suffers from inherent loss of displayed data due to the hidden line processing employed in the display unit. Epochs of high energy necessarily obscure earlier epochs (or later epochs depending on the display format) having lower energy content and thus reduce the amount of usable data available to the attending physician.
A similar display technique which does not suffer from the hidden line problem is the Density Spectral Array (DSA) display. (Shown in FIG. 2B) In this display technique, each epoch has power displayed as a dot of a given optical density. Higher power at a given frequency is signified by the placement of a darker, more dense dot while lower power is signified by a small and lighter density dot. Because no Y-axis excursions are plotted, no hidden line removal need be performed and, therefore, no data is obscured. However, inherent variability in the output device, along with the difficulties inherent in perception and quantification of shades of gray, make DSA displays imprecise and complicated to use. A commercially available electroencephalographic monitoring device which employs the DSA technique is the Cerebrotrac 2500 from SRD Advanced Instrumentation. Instead of using the density of dots to indicate levels of power, another technique is to use different colors. The color density plots are employed on the Nomad by Tracor, Inc. and also on the Cerebrotrac by SRD. Problems with color displays such as this are the same as with DSA, with the addition of their inability to be correctly interpreted by color-blind individuals (a high percent of males).
Yet another data display technique in commercial use today is that exemplified by the Cerebral Tracer from CNS Inc. (Shown in FIG. 2D) In this device, colors correspond to the four clinical frequency ranges of the EEG signal (i.e., red=delta; yellow=theta; green=alpha; blue=beta). Sixteen channels of data are simultaneously displayed as a series of pie charts superimposed over a schematic representation of the hemispheres of the patient's brain. Relative contribution of each frequency band to total signal is indicated by the included angle of the colored sector displayed for that frequency. Total power is indicated as a function of the pie chart's total area. While the instantaneous display attainable with such a technique may be useful, it lacks a trend over time feature. The manufacturer of the Cerebral Tracer instrument, therefore, also include a trend display which presents color-coded histograms for each EEG channel. While such time-displayed histograms may convey trend data, they fail to convey the full amount of data which is desirably presented to the attending physician.
Finally, the Life Scan EEG monitor from Neurometrics displays five frequency ranges in color using a modified DSA display technique. In this device, the successive epochs are displayed on a "tilted plane." (Shown in FIG. 2C) The plane gives the viewer the impression of viewing the data from above and to the right of the origin of the plot. The result of such a plot is that less information is obscured by more recent and more energetic epochs. However, the difficulties of interpretation inherent in DSA and CSA are still evident in the Life Scan device.
Despite the numerous advances in electroencephalographic signal interpretation and display, there still exists a significant need for a readily understandable display for long-term EEG data.